Colour display tubes of the type having a colour selection electrode are composed inter alia of an evacuated glass envelope consisting of a display window, a cone and a neck. A display screen composed of a large number of luminescent elements is provided on the inside of the display window. Means, for example, three electron guns, to generate three electron beams are provided opposite to the display screen in the neck of the colour display tube. In tubes of the so-called "delta" type, the three electron guns are arranged on a cone envelope according to an equilateral triangle. In tubes of the "in-line" type the axes of the three electron guns are located in one plane and the axes enclose a small angle with each other. Immediately before the display screen a colour selection electrode is provided which usually consists of a metal sheet having a large number of rows of apertures. Such a colour selection electrode is sometimes termed a shadow mask. A triplet of three luminescent elements, one red, one green and one blue luminescent element, is present behind each aperture. In tubes of the "delta" type the triplets usually consist of three phosphor dots situated at the corners of an equilateral triangle, and in tubes of the "in-line" type they usually consist of a large number of parallel phosphor stripes. Since the axes of the electron guns and hence the electron beams enclose a small angle with each other, each electron beam, considered purely theoretically, impinges only on phosphor elements of one color. In practice, however, it has been found that, as a result of tolerances in the manufacture of the colour display tubes and the deflection coils, each of the electron beams often does not impinge only on phosphor regions of one colour. In that case the colour purity of the tube is not correct. Often the convergence is also insufficient, in other words, the partial images in the three basic colours written on the display screen by the three electron guns do not coincide entirely. In many cases it has proved necessary during the manufacture of colour television sets to perform corrections so as to obtain a colour pure image and an image having a good convergence.
The colour purity and the convergence on the central part of the display screen (the so-called static convergence) are adjusted by means of a multipole unit which is arranged around or in the neck of the display tube behind the unit of deflection coils. Such colour purity units and static convergence units are described in detail inter alia in the Netherlands Patent Application No. 7707476 laid open to public inspection, and they may consist of a large number of magnetic rings which together constitute a multipole or they may consist of one ring magnetized as a multipole. The convergence on the remaining part of the display screen is termed the dynamic convergence. As a matter of fact, the electron beams must always remain converging also during the deflection over the display screen. This dynamic convergence is obtained by means of extra deflection currents through the system of deflection coils or by means of automatically converging deflection coils as described in detail in the Netherlands Pat. No. 110569. The measurement of the colour purity and the convergence has so far been carried out manually usually by means of a measuring microscope with which a raster of lines displayed on the display screen were examined. This method has a number of disadvantages and therefore other solutions were tried. One of the solutions was a device of the kind mentioned in the opening paragraph which is disclosed in the British Patent Specification No. 1,220,900. In this device the measuring microscope has been replaced by a detector which is placed against the display window and which observes a part of the display screen and which measures the colour purity. However, this device does not permit the performance of convergence measurements.
It is therefore an object of the invention to provide a device with which both the static and dynamic convergence can be measured accurately during the manufacture of colour display tubes and deflection coils.
Another object of the invention is to provide two methods for adjusting the static convergence unit of a colour display tube of the shadow-mask type by means of such a device.
A further object of the invention is to provide a device and methods which are particularly suitable for automation.